Column simultaneously focusing a particle beam and an optical beam

ABSTRACT

The invention concerns a column for producing a focused particle beam comprising: a device ( 100 ) focusing particles including an output electrode ( 130 ) with an output hole ( 131 ) for allowing through a particle beam (A); an optical focusing device ( 200 ) for simultaneously focusing an optical beam (F) including an output aperture ( 230 ). The invention is characterized in that said output aperture ( 230 ) is transparent to the optical beam (F), while said output electrode ( 130 ) is formed by a metallic insert ( 130 ) maintained in said aperture ( 230 ) and bored with a central hole ( 131 ) forming said output orifice.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to co-pendingU.S. application Ser. No. 10/239,293 filed on Sep. 20, 2002 entitled“Column Simultaneously Focusing a Particle Beam and an Optical Beam,”which application is the Section 371 (c) filing of InternationalApplication No. PCT/FR01/00812 with an international filing date of Mar.19, 2001 entitled “Column Simultaneously Focusing a Particle Beam and anOptical Beam,” which claims priority to French application No. 00/03501,filed Mar. 20, 2000 entitled “Column Simultaneously Focusing a ParticleBeam and an Optical Beam,” all of which are incorporated by reference asif fully described herein.

FIELD OF THE INVENTION

This invention relates to an optical column for simultaneously focusingan ion beam and a photon beam onto the same region.

The invention is particularly useful in the field of analysis and repairand manufacture of integrated circuits.

BACKGROUND

Focused ion beams such as ion or electron beams are currently widelyused for various types of integrated circuit analysis and manufacturingoperations, notably characterization, identification, design and failureanalysis, depassivation, vapor phase deposition, micro-machining, etc.These operations are performed using a particle beam production columndesigned to be focused onto the integrated circuit at the place intendedfor the desired intervention.

Such a column typically comprises a source of ions such as Ga+ producedfrom liquid metal which, after extraction, form an ion beam, which isthen manipulated by a focusing device comprising a certain number ofelectrodes operating at determined potentials so as to form anelectrostatic lens system adapted to focus the ion beam onto theintegrated circuit. Each electrode of the focusing device, notably theoutput electrode, consists of a series of metallic electrodes having anaperture for passage of the particle beam. It should be noted here thatthe shape of the various electrodes as well as the aperture diameterplays a determining part in aberrations, notably spherical and chromaticaberration, of the particle focusing device.

One of the limits of applying focused ion beams is the impossibility ofemploying them to provide an in-depth image of a solid. Only surfaceimages can be obtained. In the case of passivated and planarizedintegrated circuits, a surface image gives no information on theunderlying layers and circuits, which has the disadvantage of making anyintervention in the depth of the circuit extremely difficult such as, inparticular, the cutting or breaking of buried metal tracks madenecessary by design and failure analysis. To overcome this disadvantage,we employ an auxiliary light (photon) beam simultaneously and coaxiallyfocused with the particle beam. In effect, using the light beam toobtain images in the thickness of the circuits, it is possible tovisualize layers and tracks in depth and explore them, in real time,using the ion beam. It will now be understood that associating two typesof beam, an ion and a photon beam, allows the operator to bring the ionbeam exactly to the desired point on the object by means of the imagesupplied by the light beam.

Certain ion beam production columns also include an optical focusingdevice, a Cassegrain-Schwartzfeld (C-S) mirror objective lens forexample, terminating at an outlet aperture placed close to the surfaceof a sample subjected to the ion beam.

French patent 2,437,695 discloses an emission ion lens associated with aC-S type mirror objective lens. In this system, the ionic lens part, theelements of which consist of two perforated electrodes and of the sampleitself, is located between the object and the mirror objective lens. Inthis configuration, the apertures in the ion focusing device electrodesmust simultaneously be sufficiently large to provide a geometricalexpanse for the optical beam allowing sufficient sample illumination,and, relatively small so as not to deteriorate ion beam quality throughexcessive aberrations. The final diameter chosen for the outlet apertureis consequently a trade-off which is not satisfactory either for theoptical beam extent or for ion beam focusing.

Secondly, the system disclosed in French patent 2,437,695 necessitates avery small (a few millimeters) working distance and the submitting ofthe sample to an electrical field. These two constraints areunacceptable in focused ion beam technology applied to integratedcircuits: the danger of destroying the circuits by micro-electrostaticbreakdown, impossibility of slanting the sample, difficulty incollecting secondary electrons, and the practical impossibility, throughlack of space, of associating the system with a capillary tube forinjecting pre-cursor gas which is an essential accessory in focused ionbeam technology.

SUMMARY

Thus, the technical problem to be resolved by the subject matter of thisinvention is to provide a focused particle beam production columncomprising:

a device for focusing said particles carrying an output electrode havingan outlet aperture for the passage of said particle beam,

an optical focusing device for simultaneously focusing a light beam,carrying an outlet opening,

such column making it possible to associate:

a comfortable working distance of the order of 15 to 20 mm;

a final ionic lens having chromatic and spherical aberrationcoefficients of the order of magnitude of aberration coefficientsencountered in conventional ionic lenses;

a sufficient numerical aperture for the mirror optics, of the order of0.3; and

zero electric field on the object.

The solution to the technical problem posed consists, according to thisinvention, in that the outlet opening is transparent to said light beam,said output electrode being formed by a metal insert held in saidopening and carrying a central aperture forming said outlet aperture.

Thus, the column of the invention introduces independence between outletaperture diameter of the particle focusing device and outlet aperturediameter of the optical focusing device. It is thus possible to adjustcentral aperture diameter of the metal insert to an optimum value forreducing output electrode aberrations, without this in any wayprejudicing optical beam numerical aperture, the latter being determinedby the diameter of the aperture transparent to the optical beam.

According to one embodiment of the invention, provision is made for theparticle focusing device, with said particle focusing device includingan intermediate electrode, for the metal insert to project from theopening towards the intermediate electrode. In this way, if electricalbreakdown were to accidentally occur between the output electrode andthe intermediate electrode, this has maximum probability of occurring atthe metal insert, thereby protecting the means for supporting said metalinsert, notably the surface treatment of a transparent window of theoutlet aperture.

The particle production column of the invention is suited to a greatnumber of applications including:

treatment of a sample with a charged particle beam using informationsupplied by the optical beam and, in particular, precise investigationof the effects of a particle beam on an integrated circuit by means ofinformation supplied by the optical beam,

treatment of a sample requiring use of a laser focused onto said sampleand, in particular, removal of integrated circuit layers by laser withor without chemical assistance, allowing etching or milling at a finerand more local scale, assisted deposition, or electron or ion beamanalysis,

integration of electron or ion beams with infra-red microscopy forintegrated circuit analysis,

laser chemical etching allowing milling of integrated circuits by ionicbeam or electron beam probing,

display of optical transitions created, for example, by the effect ofion beams or other light phenomena appearing on a sample,

laser marking of integrated circuits,

electron beam probing of diffusion in integrated circuits or othersamples,

canceling of the effects of static charges by UV photons when performingfocused electron or ion treatment,

spectroscopic micro-analysis of photons emitted under particle impact.

The description that follows with reference to the attached drawings,provided by way of non-limiting example, will lead to a betterunderstanding of the invention and how it may be carried out.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial side view in section of a particle beam productioncolumn according to one first embodiment of the invention.

FIG. 2 is a partial side view in section of a particle beam productioncolumn according to a second embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In FIG. 1, we have partially shown, in section, a particle beamproduction column for focusing onto an integrated circuit 1. Theparticle beam axis which coincides with the column axis is identified byreference letter A. Although the column in FIG. 1 applies to all sortsof charged particles, electrons or ions, we shall take below the exampleof an ion beam.

Only the downstream part of the column is shown in FIG. 1, the ionsource and the means for extracting and conditioning the ion beam whichare known per se, not being shown.

The part of the column shown in FIG. 1 essentially comprises a device100 for focusing the ion beam onto integrated circuit 1. This device 100carries three electrodes, specifically an input electrode 110 which isgrounded, an intermediate electrode 120 brought to a nonzero potential Vwhich may be positive or negative for example of 20 Kev, and an outputelectrode 130 also grounded. These electrodes 110, 120, 130 arecontained between lateral walls 140 of the column, the latter beinggrounded.

In fact, on FIG. 1 it can be seen that intermediate electrode 120 is acomplex two-part electrode made up by a first intermediate electrode 121arranged close to input electrode 110 and by a second intermediateelectrode 122 arranged close to output electrode 130. These electrodestogether form an electrostatic lens of the thick, geometricallyasymmetric but electrically symmetric type.

It can be seen on FIG. 1 that an optical focusing device 200 designed tofocus an optical beam F simultaneously and coaxially with the particlebeam on axis A is located between the two intermediate electrodes 121,122. This device 200 allows both optical beam F to be focused ontosample 1 thereby forming an enlarged image of the sample as well ascollection of light radiation emitted by said sample or by sputteredatoms following ionic bombardment. Optical beam F is obtained from anon-illustrated light source generally arranged laterally with respectto the column with the light being re-directed parallel to axis A and bya mirror at 45.degree. located on said axis and including an aperturefor passage of the ion beam.

In the embodiment of FIG. 1, optical focusing device 200 is aCassegrain-type mirror objective lens comprising a first convexspherical mirror 210 located in optical beam path F and a second concavespherical mirror 220 focusing onto integrated circuit 1 the beam comingfrom first mirror 210. The latter includes an aperture 211 for allowingthe ion beam to pass through the second intermediate electrode 122, theassembly formed by the first mirror 210 and said second intermediateelectrode 122 being held at the centre of the column by a metal tripod212 providing a high degree of transparency to the light beam.

As can be seen in FIG. 1, optical focusing device 200 also carries anoutlet aperture 230 itself including a window 240 that is transparent tophotons of optical beam F, held by its edges to the outer housing of thegrounded column. Output electrode 130 is formed by a metal insertpassing through a window 240, and which is retained by said window 240and including a central aperture at its middle 131 for the output ofelectrode 130. In order to ground said output electrode 130, transparentwindow 240 is electrically conducting. In particular, it can beglass-plated covered with at least one conducting layer 241 such asindium and/or tin oxide. It is thus possible to select a small diameteroutlet for aperture 131, compatible with the resolution desired for theion beam, while maintaining, in an independent fashion, a largerdiameter opening 230, providing a geometrical expanse for the opticalbeam ensuring sufficient numerical aperture and thereby obtaining a highquality optical image of the sample 1 observed. Clearly, outlet window240 could just as well be made of any bulk material transparent tophotons, and electrically conducting.

In FIG. 1 it can be seen that metal insert 130 projects from the surfaceof window 240 towards second intermediate electrode 122, therebyprotecting said window in the case of electrical breakdown, the latteroccurring between insert 130 and the second electrode 122.

Like the embodiment shown in FIG. 1, optical focusing device 200 of theembodiment of the invention shown in FIG. 2 is a Cassegrain-typeobjective lens with mirrors 210, 220 brought to a high-voltagecomprised, for example, between 10 and 20 keV.

However, a first mirror 300 is located on ion beam axis A between thefirst intermediate electrode 121 and the second intermediate electrode122 and, more precisely, between first intermediate electrode 121 andthe Cassegrain-type objective lens with mirrors 210, 220. This mirror300 carries an aperture 310 for passage of the ion beam. It is inclinedsubstantially at 45.degree. with respect to axis A in order to deflectoptical beam F through about 90.degree. laterally towards a secondmirror 320 arranged in the space comprised between the lateral walls 140of the column and part 120. This second mirror 320 is itself angled at45.degree. with respect to axis A. It deflects beam F through 90 degreesin the same direction as axis A, parallel to the latter.

Thus, the diameter of aperture 111 provided at the extremity of inputelectrode 110, designed to allow passage of ion beam A but the functionof which is not, contrary to the embodiment of FIG. 1, to allow passageof the optical beam, can be reduced to values of millimetric scaleorder. Further, deflector plates 10 located upstream of input electrode110 no longer require the anti-reflection treatment needed for goodconduction of the optical beam. Finally, artefacts due to the light beaminteracting with the walls of the ionic optical elements which did existupstream of first mirror 300, in particular at deflector plates 10 ofthe embodiment in FIG. 1 and which notably decrease quality ofinterpretation of the images obtained, are eliminated.

Further, in the embodiment of FIG. 2, aperture 230 does not carry awindow 240 but rather a set of metallic or, at the leastelectrically-conducting, tabs or legs. There are for example three suchtabs forming a metallic tripod 250 which is retained by the edges of theouter housing of the grounded column, delimiting aperture 230. Theyensure good retention of insert 150 while ensuring aperture 230 is kepttransparent for the optical beam. Thus, like in the embodiment of FIG.1, it is possible to choose, for output aperture 131, a small value ofdiameter compatible with the resolution required for the ionic beam,while maintaining, completely independently, a larger diameter aperture230 offering the light beam a geometrical expanse allowing sufficientillumination of the observed sample 1.

Finally, in the embodiment of FIG. 2, the tabs or legs of metal tripod212 designed to hold the unit formed by mirror 210 and the secondintermediate electrode 122 are curved so as to increase their spacingfrom the legs of tripod 250 and output electrode 130. Thanks to this,risks of spark-over are limited as are distortions of the electricalfield due to the tripod.

1. A column for simultaneously producing a focused particle beam and afocused light beam, the column comprising: means for generating acharged particle beam; means for focusing the charged particle beam on adevice under test operably associated with the means for generating acharged particle beam; and means for optically focusing an opticalviewing apparatus on the device under test, the means for opticallyfocusing operably coupled with the means for focusing the chargedparticle beam.